The heating value of natural gas has a significant impact on its monetary value. In general, the heating value of natural gas increases as the concentration of low volatility, high molecular weight components increases. Condensation of gas phase components, which reduce the proportion of high molecular weight components, therefore tends to decrease gas phase heating value, while vaporization of entrained liquid has the opposite effect.
In order for natural gas supply to keep up with demand over the next 10 to 20 years, it will be necessary to increase production from deep-water fields in the Gulf of Mexico. (Refer to Volume 1, Fall/Winter 1997 official newsletter of Colorado Engineering Experiment Station Inc.) Gas produced from deep-water fields contains higher concentrations of low volatility components, such as water vapor and heavy hydrocarbons, and has a higher susceptibility to condensation than shelf and onshore production gas.
Additionally, some onshore produced gas, particularly in low ambient temperature regions, frequently contains entrained liquids. Other liquids, which can influence vapor phase composition when fluid pressure or temperature changes occur, include glycols and amines, which are carried over into the gas phase from gas contactors designed to remove water vapor and acid gases, respectively.
A Joint Industry Project (JIP) is underway to address problems associated with measurement and transportation of wet gases. A part of the JIP focus will include improvement of wet gas sampling techniques.
The American Petroleum Institute (API) and the Gas Processors Association (GPA) are two leading industry organizations, having recommended standard practices for sampling and analysis of natural gas.
Both of these organizations recommend the use of sample probes inserted into the process fluid, for the purpose of extracting samples of said process fluids. Further, both require that the probe be inserted to a specific depth in the containment vessel or pipeline. (Refer to Manual of Petroleum Measurement Standards chapter 14—Natural Gas fluids measurement, section 1 collecting and handling natural gas samples for custody transfer, fourth edition, August 1993.)
Insertion of probes into pressurized systems for collecting liquid samples is also a frequent requirement. The sample probe is generally the first element, as well as being a key component of a sample conditioning system. The accuracy of the fluid sample's compositional analysis can be impacted by the sample probes performance. The sample probe may provide an extracted sample which may be transported to a process analyzer or the like, but it may also be utilized to direct a sample into a cylinder (called “cylinder sampling”) where the sample is container for later analyzation or other use.
The sample may be referenced as a “representative sample”, however this does not always mean that all components of the sample stream are ultimately present for testing. Often “representative” can relate only to certain components of interest or phase. This is the case with Natural Gas process sampling, where a representative sample under industry standards is considered to be components in the gas phase. Thus, liquid is typically intentionally excluded at pipeline condition sampling. Also, applications where a liquid is entrained in a process gas in droplet form present other problems with regard to sample extraction representative of components of interest, as liquid droplets in the sample train can alter the composition of the gas phase, as when a combined gas/liquid sample stream undergoes pressure and/or temperature changes. If the desired representative sample is not accurately obtained, the analytical process is impacted.
Thus, it is advantageous under certain conditions to extract only the gas phase at the prevailing pressure and temperature of the source gas.
In many cases, the cost of installing a fixed probe at each sample location is cost prohibitive. For example, some pipeline companies sample fluids at several thousand locations. Outfitting each sample tap location could cost several million dollars. The result is that fluids are often sampled without the use of probes, which results in non-conformance of applicable standards, and inaccurate sample analysis.
It would be desirable, therefore, to have the capability of inserting a probe into the pressurized fluid systems at the time of sampling, (and preconditioning where desired) and retracting said probe upon the completion of the sampling process. To be effective, the probe insertion/retraction process must be safe, easy and quick to perform, portable, and effective for the intended service.
The same can be said for measuring the fluid temperature, wherein a temperature probe or well designed to receive a temperature probe is required to be inserted and/or retracted from a pressurized fluid stream or containment vessel. There is also a frequent need to insert other types of devices into pressurized system, such as the insertion/retraction of corrosion coupons, flow measuring devices and various types of sensors, analyzer, and devices.
Additionally, it is often desirable to retract a probe-type of device from a pressurized system to accommodate “pigging”, or other type of maintenance operation.
Insertion and retraction devices for insertion/retraction of probe or probe like devices are known. However, they all employ a seal, through which the probe is inserted into the pressurized system, for the purpose of preventing pressurized fluid from leaking.
In these probes, the insertion force is derived either from a screw-type of device, or pneumatically or hydraulically. Such is the case with U.S. Pat. Nos. 4,177,676, 5,770,809, 5,639,975 and 5,627,749. The apparatus of these aforementioned patents are bulky and long, requiring, at a minimum, a length of at least twice the maximum insertion length to extend above the point of insertion into a vessel. In many cases, such as in the tight quarters of a chemical plant, refinery, or offshore drilling platform, the bulk and length of these type devices preclude their use. Obviously, their design does not lend itself to rapid and safe insertion and retraction from a pressurized fluid source.